The present invention is directed to integrated circuits. More particularly, the invention provides amplification systems and methods with noise reductions. Merely by way of example, the invention has been applied to a Class-D amplifier. But it would be recognized that the invention has a much broader range of applicability.
Usually, a switching amplifier (e.g., a Class-D amplifier) is an amplifier where output transistors are often operated as switches. The Class-D amplifier is widely used in audio amplification, and has power-efficiency advantages over certain linear audio-amplifier classes such as Class A, B, and AB.
FIG. 1 is a simplified conventional diagram showing an amplification system using a Class-D amplifier. The amplification system 100 includes a modulator 102, two transistors 104 and 106, an inductor 108, a resistor 110, two capacitors 112 and 114, and an output load 116. For example, the transistor 104 is a P-channel metal-oxide-semiconductor field effect transistor (MOSFET), or an N-channel MOSFET. In another example, the transistor 106 is an N-channel MOSFET. In yet another example, the output load 116 is a speaker. In yet another example, the inductor 108 and the capacitor 112 are included in a low pass filter. In yet another example, the modulator 102, and the transistors 104 and 106 are included in a Class-D amplifier 130.
The modulator 102 receives an input audio signal 118, and generates gate drive signals 120 and 122. For example, the gate drive signals 120 and 122 are the same or different. In yet another example, pulse widths of the gate drive signals 120 and 122 are related to the amplitude of the input audio signal 118. In yet another example, if the amplitude of the input audio signal 118 is constantly zero, the modulator 102 outputs the gate drive signals 120 and 122 at a 50% duty cycle. In yet another example, if the amplitude of the input audio signal 118 has a proper positive value, the modulator 102 outputs the gate drive signal 120 at a close to 100% duty cycle, and the modulator 102 outputs the gate drive signal 122 at a close to 0% duty cycle. In yet another example, if the amplitude of the input audio signal 118 has a proper negative value, the modulator 102 outputs the gate drive signal 120 at a close to 0% duty cycle, and the modulator 102 outputs the gate drive signal 122 at a close to 100% duty cycle.
The gate drive signals 120 and 122 are then received by the transistors 104 and 106, respectively. The transistors 104 and 106 in response generate an output voltage signal 124 (e.g., Vout), and an output current 128 (e.g., IL). For example, the gate drive signals 120 and 122 are logic control signals, and hence the transistors 104 and 106 operate like switches. In another example, the transistor 104 is turned on by the gate drive signal 120, and the transistor 106 is turned off by the gate drive signal 122. Then, the output voltage signal 124 is equal to a positive power supply voltage (e.g., PVCC). The output current 128 increases, and energy is delivered to the output load 116.
In yet another example, the transistor 104 is turned off by the gate drive signal 120, and the transistor 106 is turned on by the gate drive signal 122. Then, the output voltage signal 124 is equal to a negative power supply voltage (e.g., ground). The output current 128 decreases in magnitude, or the flow direction of the output current 128 is reversed. In yet another example, the output voltage signal 124 (e.g., Vout) is a pulse signal that has a same duty cycle as the gate drive signal 120 or 122.
The low pass filter including the inductor 108 and the capacitor 112, along with the resistor 110 and the capacitor 114, receives the output voltage signal 124 and the output current 128. In response, an output audio signal 126 is generated to drive the output load 116. For example, the output audio signal 126 is approximately equal to the input audio signal 118.
But in some situations, the Class-D amplifier 130 generates undesired audible transients (e.g., clicks and/or pops), which are often related to the start or stop of PWM switching, offsets of output direct current (DC), and transients of power supply. For example, the Class-D amplifier 130 generates the undesired audible transients if the amplification system 100 changes its operation mode. In another example, the Class-D amplifier 130 generates the undesired audible transients during the process of powering up the amplification system 100 or the process of powering down the amplification system 100. In yet another example, the Class-D amplifier 130 generates the undesired audible transients if the amplification system 100 enters a mute mode (e.g., the gain of the Class-D amplifier 130 is zero) or an un-mute mode (e.g., the gain of the Class-D amplifier 130 is not zero).
FIG. 2 is a simplified conventional diagram showing certain components of the Class-D amplifier 130 as part of the amplification system 100. The amplification system 100 includes the two transistors 104 and 106, the inductor 108, the resistor 110, the two capacitors 112 and 114, and the output load 116. Further, the amplification system 100 includes a loop filter 202, a pulse width modulation (PWM) signal generator 204, and a logic and gate driver 206. For example, the loop filter 202, the PWM signal generator 204, and the logic and gate driver 206 are included in the modulator 102. In another example, the loop filter 202, the PWM signal generator 204, the logic and gate driver 206, and the transistors 104 and 106 are included in the Class-D amplifier 130.
The loop filter 202 receives an input audio signal 208, and generates in response an output signal 210. The PWM signal generator 204 receives the output signal 210 as well as a ramping signal 212, and generates in response a PWM signal 214. The logic and gate driver 206 receives the PWM signal 214, and generates two gate drive signals 216 and 218. For example, the ramping signal 212 has a triangle waveform. In another example, the gate drive signals 216 and 218 are the same or different. In yet another example, pulse widths of the PWM signal 214 are proportional to the amplitude of the input audio signal 208. In yet another example, pulse widths of the gate drive signals 216 and 218 are proportional to the amplitude of the input audio signal 208.
The gate drive signals 216 and 218 are then received by the transistors 104 and 106, respectively. In response, the transistors 104 and 106 generate an output signal 220. For example, if the PWM signal 214 is at a logic high level, the transistor 104 is turned on by the gate drive signal 216, and the transistor 106 is turned off by the gate drive signal 218. Then, the output signal 220 is equal to a positive power supply voltage (e.g., PVCC). In another example, if the PWM signal 214 is at a logic low level, the transistor 104 is turned off by the gate drive signal 216, and the transistor 106 is turned on by the gate drive signal 218. Then, the output signal 220 is equal to a negative power supply voltage (e.g., ground).
The low pass filter including the inductor 108 and the capacitor 112, along with the resistor 110 and the capacitor 114, receives the output signal 220. In response, an output audio signal 222 is generated to drive the output load 116. For example, the output audio signal 222 is approximately equal to the input audio signal 208. In another example, a DC voltage or an averaged voltage across the capacitor 114 is often required to be approximately equal to half of the positive power supply voltage, e.g., PVCC/2, in order for the amplifier 130 to operate properly. The voltage across the capacitor 114 is often referred to as an output common-mode voltage level.
The output signal 220 is received by the loop filter 202 as a negative feedback signal to correct non-linearity and errors in order to improve performance of the amplification system 100 (e.g., reduce distortions). Also, the output signal 220 is filtered by the low pass filter including the inductor 108 and the capacitor 112. For example, the output signal 220 often contains harmonics in a wide frequency range, considering that the switching frequency of the amplifier 130 usually ranges from several hundred kHz or even above 1 MHz. In another example, the low pass filter including the inductor 108 and the capacitor 112 is often needed to recover the input audio signal 208 for an audio range of 20-22 kHz.
Some noise-reduction techniques are often used to reduce the undesired audible transients generated during the process of powering up or powering down the amplification system 100. For example, an input common-mode DC ramping signal is applied so that the output common-mode DC signal follows the input common-mode DC ramping signal and slowly charges the capacitor 114, in order to achieve the ramping up of the output common-mode DC signal. Such ramping up of the output common-mode DC signal, along with the gain ramping-up, is used to reduce the undesired audible transients during the process of powering up the amplification system 100. In another example, ramping down of an output common-mode DC signal, along with a gain ramping-down, is used to reduce the undesired audible transients during the process of powering down the amplification system 100. In yet another example, a gain ramping-up is used to reduce the undesired audible transients if the amplification system 100 switches from the mute mode to the un-mute mode. In yet another example, a gain ramping-down is used to reduce the undesired audible transients if the amplification system 100 switches from the un-mute mode to the mute mode.
But, even though the above-mentioned noise-reduction techniques are applied, the undesired audible transients may still be generated in some situations. For example, if the Class-D amplifier 130 has a switching frequency of several hundred kHz, cycle skipping is usually observed at a low common-mode DC ramping level. The cycle skipping often causes the switching frequency of the amplifier 130 to fall into a range of audible frequencies, and hence the undesired audible transients (e.g., clicks and/or pops) can be generated.
Hence it is highly desirable to improve the techniques of noise reduction in amplification systems.